Many enclosures that contain sensitive instrumentation must maintain very clean environments in order to keep that instrumentation running properly. Examples include enclosures with sensitive optical surfaces, electronic connections and magnetic data storage surfaces on computer hard disk drives. Contaminants to these surfaces may be either particulate or gaseous in nature insomuch as they interfere with the proper operation of the equipment, and may enter the enclosure from the outside environment or be generated within the enclosure such as by outgassing of components.
Filtration devices to keep particulate from entering these enclosures are well known. They may consist of a filtration media held in place by a housing of polycarbonate, acrylonitrile butadiene styrene, or some other material; or they may consist of a filtration media in the form of a self-adhesive disk utilizing a layer or layers of pressure sensitive adhesive. These devices are mounted and sealed over a vent hole in the enclosure to filter the air entering the enclosure. Filtration performance depends not only on the filter having a high filtration efficiency but also on having a low resistance to air flow so that unfiltered air does not leak into the enclosure through a gasket or seam instead of entering through the filter.
Cartridges that contain adsorbents or reactants to remove gas or vapor impurities are also well known. They may consist of an adsorbent material held in place by a housing of polycarbonate, acrylonitrile butadiene styrene, or other material which also utilizes a filtration media that allows the exchange of gases in and out of the adsorbent cartridge while preventing the adsorbent material from becoming a source of particulate contamination. A preferred configuration of the adsorbent cartridge is to have the adsorbent completely encapsulated by the filtration media such as in a tube of pure expanded polytetrafluoroethylene as described in U.S. Pat. No. 4,830,643.
Another constraint, however, in many enclosures is space. The sensitive instrumentation is continually being miniaturized and put in smaller and smaller enclosures. In some cases this compounds the contamination problem as surfaces become more sensitive and closer together such as in computer hard disk drives where particulates, hydrocarbon gases, acid gases, and solvent vapors become more of a problem as read/write head flying heights become smaller and more sensitive higher density thin film recording media are employed.
Breather filters that are constructed of only filter media and a self-stick adhesive are ideal for these applications as they can be made very thin and small in size. Adsorbent cartridges or tubes, however, take up valuable space in these enclosures, and when they are used, they often must be put into a corner away from the most sensitive surfaces because of space constraints.
One solution of these problems is the combination of an adsorber breather filter. These can be made by filling a cartridge of polycarbonate, acrylonitrile butadiene styrene, or some other material with adsorbent and securing filter media on both ends of the cartridge and attaching said cartridge to the enclosure which needs a controlled environment. This allows air to enter the enclosure through the adsorbent to clean the air that enters the enclosure. Gas or vapor contaminants that outgas or originate from sources inside the enclosure can be captured by the adsorbent by diffusing through the filter media into the adsorbent material. These cartridges also take up space although they can be mounted outside the enclosure. Outside mounting, however, raises problems of rigidity and sturdiness as a filter that protrudes from the enclosure is subject to easier damage.
A second contamination adsorbent breather filter is also available that encapsulates the adsorbent material between two layers of filter media and is applied to the enclosure with a layer of self-stick adhesive.
Both of these above-mentioned adsorbent breather configurations, however, have two major drawbacks. First, since they utilize two layers of filter media and a layer of adsorbent material, they suffer from a fairly high resistance to air flow. As mentioned earlier, filtration performance in a breather filter depends in part upon the filter having a low resistance to air flow such that unfiltered air is not forced or allowed to leak through gaskets or seams that might open up under higher pressure. Secondly, since they are located directly under or over a vent hole into the enclosure, one side of the adsorbent breather faces the outside atmosphere and the adsorbent can become more quickly spent or saturated with gaseous contaminants.
Moreover, since these breather assemblies are mounted over holes in the enclosure, they fail to provide adequate shielding to electromagnetic interference. Further, no indicator is present on these assemblies to let the user know when the filter adsorbent has been spent.
There is a need for a system that minimizes space requirements for adsorbent, and has a long lasting adsorbent that is easily mounted proximate to the critical areas of enclosures housing sensitive instrumentation.